The    Atomic    Weight 

of 

Palladium 


THESIS 

Presented  to  the  Faculty  of  the  Department  of  Philosophy 

of  the 

University   of  Pennsylvania 
in  Partial  Fulfilment  of  the  Requirements 

•for  the  Degree  of 
Doctor  of  Philosophy, 

BY 

GEORGE  IRVING  KEMMERER 


1908 


HODGES  &  KIRK 

PHILADELPHIA 

1908 


OF  THE 

(    UNIVERSITY  ) 

OF 


This  work  was  undertaken  at  the  suggestion  of  Dr.  Edgar 
F.  Smith  and  I  wish  to  express  my  most  sincere  thanks  for 
his  many  suggestions  and  unfailing  kindness. 


THE  ATOMIC  WEIGHT  OF  PALLADIUM. 

The  atomic  weight  of  palladium  has  been  the  subject  of  no 
less  than  twelve  separate  investigations.  The  results  vary 
so  widely  that  it  seemed  advisable  to  give  it  still  further  con- 
sideration. 

The  previous  work  is  most  easily  summed  up  by  means  of 
the  following  table : 

Salt  used. 

1  Pd 

2  KsPdCU 

3  K2PdCl4 

4  Pd(NH3Cl)2 

5  Pd(NH3Cl)2 


Date.          Author. 
1826  Berzelius 
1828  Berzelius 
1847  Quintus  Icilius 
1889  Keiser 
1892  Bailey  and  Lamb 


1892  Keller  and  Smith    6     Pd(NH3Cl)2 

1893  Joly  and  Leidie      7    K^PdCh 

1894  Keiser  and  Breed  8    Pd(NH3Cl)a 
1899  Hardin  9     (NH4)2PdBr* 

Pd(C6H5NH2Cl)2 
Pd(C«H5NH2Br)2 
1906  Krell  10    Pd(NH3Cl)3 

1906  Amberg  11    Pd(NH3Cl)a 

1907  Woernle  12    Pd(NH3Cl)a 


Determined. 
PdS 
Pd-Cl2-KCl 
Pd. 
Pd 
Pd  and  C12 
Pd 

Pd 

106.5 

106.54 
I0573I 
107.19 
105.71 
106.518 

H±si. 

in.8 

106.31 

10545 
106.91 
105.92 
106.25 

Pd 
Pd 
Pd 


107.01 

106.69 

106.7 

106.71 


106.74 


1.  Berzelius.  Pogg  Ann.  8,  180,  1826. 

2.  Pogg  Ann.  13,  454,  1828. 

3.  Quintus  Icillius.  Clark.  Atomic  Weights,  315. 

4.  Keiser.  Amer.  Chem.  J.  n,  398.   1889. 

5.  Bailey  and  Lamb,  J.  Chem.  Soc.  61,  745,  1892. 

6.  Keller  and  Smith,  Amer.  Chem.  J.  14,  423,  1892. 

7.  Joly  and  Leide,  Compt.  Rend.  116,  146,  1893. 

8.  Keiser  and  Breed,  Amer.  Chem.  J.  16,  20,  1894. 

9.  Hardin.  J.  Am.  Chem.  Soc.  21,  943,  1889. 
10.  Krell,  Dissertation,   Erlangen,   1906. 

n.  Amberg,  Lieb.  Ann.  341,  235,  1905. 

12.  Woernle  Dissertation,   Erlangen,   1907. 

Purification  of  Materials. 

The  metal  was  obtained  from  Baker  and  Adamson  as  sheet 
palladium  and  weighed  32  grams.  Sample  N6.  I  was  purified 
according  to  the  method  of  Keller  and  Smith1  and  was 
used  to  prepare  salt  (A).  Sample  No.  2  was  treated  similarly, 
but  the  use  of  mercuric  cyanide  was  eliminated  and  sodium 
and  potassium  salts  were  replaced  by  similar  ammonium  com- 
pounds wherever  possible. 

The  complete  process  was  as  follows.  The  metal  was  dis- 
solved in  aqua  regia  and  repeatedly  evaporated  with  hydro- 
chloric acid  to  remove  the  nitric  acid.  The  residue  was  then 
(i)  Keller  and  Smith,  Amer.  Chem.  J.  14-423,  1892. 


186957 


taken  up  in  a  little  dilute  hydrochloric  acid  and  filtered.  The 
concentrated  solution  of  palladous  chloride  was  treated  with 
an  excess  of  ammonia  and  warmed.  The  palladium  dissolved 
leaving  a  brownish  residue  of  ferric  hydroxide  and  an  insoluble 
rhodium  compound.  These  were  filtered  out,  the  solution 
heated  to  expel  the  excess  of  ammonia  and  when  cool,  hydro- 
chloric acid  was  added  until  just  acid.  The  pallado-diammo- 
nium  chloride  separated  in  small  bright  yellow  crystals.  These 
were  washed  by  decantation,  filtered  on  a  suction  filter,  washed 
with  cold  water  and  dissolved  in  a  little  cold  dilute  ammonia. 
A  trace  of  an  insoluble  rhodium  compound  remained  on  the 
filter.  The  filtrate  was  now  heated  to  expel  the  excess  of  am- 
monia and  the  pallado-diammonium  chloride  reprecipitated 
by  acidifying  with  hydrochloric  acid.  This  precipitate  was 
washed,  dried  and  carefully  ignited  in  the  air  to  the  green 
oxide.  The  oxide  was  then  placed  in  a  rose  crucible  and  re- 
duced in  hydrogen  to  the  metal,  which  was  dissolved  in  aqua 
regia  and  repeatedly  evaporated  with  hydrochloric  acid.  The 
chloride  remaining  was  taken  up  in  a  little  dilute  hydrochloric 
acid,  diluted  to  two  liters  and  nearly  neutralized  with  sodium 
carbonate.  The  solution  was  then  treated  with  an  equivalent 
quantity  of  ammonium  cyanide  and  the  resulting  yellow  palla- 
dium cyanide  was  washed  by  decantation  for  one  week.  It 
was  filtered  by  means  of  an  invert  filter  and  dried.  The  dry 
cyanide  was  placed  in  an  unglazed  porcelain  crucible,  con- 
tained in  a  similar  larger  crucible.  Each  crucible  was  covered 
and  the  whole  heated  at  the  highest  temperature  of  the  muffle 
furnace  for  six  hours.  The  resulting  metal  was  treated  with 
concentrated  hydrochloric  acid  to  remove  any  oxide  of  copper 
and  again  dissolved  in  aqua  regia.  The  nitric  acid  was  re- 
moved by  evaporation  with  hydrochloric  acid  and  the  chloride 
dissolved  with  a  little  dilute  hydrochloric  acid  and  diluted  to 
two  liters.  This  solution  was  saturated  with  sulphur  dioxide, 
treated  with  ammonium  sulpho-cyanide  and  allowed  to  stand 
for  two  days,  but  no  precipitate  was  formed.  The  sulphur 
dioxide  was  now  expelled  by  heating  on  the  waterbath  when 
the  solution  returned  to  its  original  color  and  a  little  palla- 
dium was  precipitated.  This  was  filtered  out ,  the  solution 
treated  with  ammonium  formate  and  nearly  neutralized  with 
ammonia.  On  heating  the  solution  the  metal  was  almost 
completely  precipitated.  The  black  palladium,  filtered  out 
and  thoroughly  washed,  was  dried  and  heated  in  a  double 
crucible,  similar  to  the  one  used  for  the  cyanide,  to  the  high- 


est  heat  of  the  muffle  for  six  hours.  When  cool,  the  slight 
tarnish  of  oxide  was  removed  by  treating  with  hydrochloric 
acid,  after  which  the  metal  was  silver  white  in  color. 

Water.  The  water  for  the  preparation  of  salt  A  was  only 
distilled  once.  For  all  the  other  work  it  was  redistilled  from 
glass  using  a  block  tin  condenser  and  Jena  glass  receiving 
flasks. 

Hydrochloric  Acid.  Hydrochloric  acid  was  diluted  and  dis- 
tilled from  an  apparatus  constructed  entirely  of  glass.  The 
first  and  last  portions  of  the  distillate  were  rejected,  only  the 
middle  portion  being  used. 

Sodium  Carbonate.  So-called  pure  carbonate  of  soda  was 
recrystallized  once  from  porcelain  and  three  times  from  plat- 
inum, drying  each  time  centrifugally. 

Mercuric  Cyanide.  Mercuric  oxide  was  first  prepared  by 
treating  a  solution  of  sublimed  mercuric  chloride  with  sodium 
hydroxide  made  from  sodium.  This  was  thoroughly  washed 
and  dissolved  in  hydrocyanic  acid,  made  by  distilling  the  fresh- 
ly prepared  anhydrous  acid  into  water.  The  solution  was  then 
evaporated,  and  allowed  to  crystallize.  The  salt  was  recrystal- 
lized from  water  and  dried  centrifugally. 

Ammonium  Formate.  Formic  acid  was  distilled,  diluted 
and  neutralized  with  ammonia  gas.  The  solution  was  then 
evaporated  when  ammonium  formate  crystallized  out. 

Sulphur  dioxide.  Dioxide  of  Sulphur  was  made  by  treat- 
ing a  solution  of  sodium  aci.d  sulphite  with  concentrated  sul- 
phuric acid.  The  gas  was  washed  with  water  and  sulphuric 
acid. 

Ammonium  Sulpho  cyanate.  Sulpho  cyanate  of  ammonium 
was  crystallized  from  alcohol,  then  recrystallized  from  water. 

Preparation  of  Salts.  Salt  A,  pallado-diammonium  chloride, 
was  prepared  from  the  sample  of  pure  metal  No.  i.  It  was 
dissolved  in  aqua  regia  and  evaporated  to  dryness  five  times 
with  hydrochloric  acid.  The  chloride  was  finally  taken  up 
with  as  little  dilute  hydrochloric  acid  as  possible,  the  solution 
filtered  through  a  hardened  filter  and  treated  with  an  excess 
of  ammonia  gas.  When  this  was  warmed  the  red  pallado-am- 
monium  salt  went  completely  into  solution.  An  excess  of 
hydrochloric  acid  was  then  added  to  the  ammoniacal  solution, 


when  the  pallado-ammonium  chloride  separated  in  small  yel- 
low crystals.  These  were  filtered  out  on  a  hardened  filter  and 
washed  with  cold  water.  After  being  removed  from  the  moist 
filter  they  were  dried  over  sulphuric  acid,  then  powdered  and 
brought  to  constant  weight  by  heating  at  no°C. 

Salt  B.  The  pallado-diammonium  chloride  lettered  B  was 
prepared  similarly  to  A  except  that  the  metal  used  was  part 
of  sample  No.  2. 

Salt  C.  The  pallado-diammonium  cyanide  Pd(NH3CN)2 
was  prepared  from  metal  No.  2.  The  metal  was  dissolved  in 
aqua  regia,  freed  from  nitric  acid  and  the  chloride  dissolved  in 
dilute  hydrochloric  acid.  This  solution  was  then  diluted  and 
treated  with  an  equivalent  quantity  of  ammonium  cyanide  con- 
taining a  slight  excess  of  ammonia.  By  this  method  the  cyan- 
ide was  precipitated  almost  completely  without  neutralizing 
the  hydrochloric  acid  with  sodium  carbonate.  The  yellow 
gelatinous  cyanide  was  washed  by  decantation  for  two  weeks 
which  completely  freed  it  from  chlorine.  The  water  was  then 
removed  as  completely  as  possible  and  ammonia  gas  conducted 
into  the  suspended  cyanide.  It  dissolved  readily  and  com- 
pletely. The  excess  of  ammonia  was  expelled  by  warming 
on  the  water  bath  and  on  cooling  the  pallado-diammonium 
cyanide  crystallized  in  small,  beautiful  white  crystals.  These 
were  separated  on  a  hardened  filter,  and  thoroughly  washed 
with  water.  They  were  then  dried  over  phosphorus  pentoxide 
and  powdered.  The  salt  was  brought  to  constant  weight  by 
heating  to  45°C  in  a  vacuum  desiccator  over  phosphorus  pent- 
oxide.  A  trial  sample  of  the  salt  was  treated  in  a  desiccator 
heated  by  a  small  incandescent  lamp.  Constant  weight  was 
obtained  after  two  weeks'  heating,  but  the  salt  darkened  very 
slightly  on  the  surface.  In  order  to  overcome  this  difficulty 
the  desiccator  was  coated  with  black  varnish  and  heated  by 
a  coil  of  resistant  wire  carrying  a  current  of  0.4  of  an  ampere. 
This  gave  a  temperature  varying  from  43  to  46°  and  the  salt 
was  constant  in  weight  after  two  weeks'  heating. 

Weighing. 

All  weighings  were  made  on  a  Troemner  balance  which  has 
been  used  exclusively  for  atomic  weight  work  in  this  labora- 
tory. It  was  sensitive  to  .02  of  a  milligram  and  duplicate 
weighings  of  the  same  thing  seldom  varied  more  than  .03  of  a 
milligram.  The  brass  and  platinum  weights  were  carefully 
calibrated  and  checked  during  the  year.  The  salt  to  be  re- 


duced  was  weighed  in  a  porcelain  boat  contained  in  a  weigh- 
ing bottle.  These  were  counterbalanced  by  a  similar  tube 
and  boat  of  equal  weight.  All  weights  were  reduced  to  the 
vacuum  standard  using  the  following  specific  gravities :  Pal- 
ladium metal,  11.45;  pallado-diammonium  chloride,  2.55;  pal- 
lado-diammonium  cyanide,  2.5.  During  the  course  of  the  ex- 
periments the  glaze  on  the  boat  gradually  darkened,  probably 
due  to  the  absorption  of  minute  traces  of  palladium.  Platinum 
boats  were  tried,  but  it  was  not  possible  to  heat  them  suffi- 
ciently high  without  danger  of  alloying.  To  eliminate  any 
possible  error  from  the  absorption  of  palladium,  after  each 
experiment  the  boat  was  boiled  in  aqua  regia  and  soaked  in 
hot  water.  It  was  then  dried  and  heated  to  bright  redness 
in  hydrogen  for  one  hour.  After  cooling  in  nitrogen  it  was 
removed  to  the  desiccator  and  allowed  to  stand  in  the  balance 
room  for  two  hours  before  weighing.  While  using  this  meth- 
od the  weight  of  the  boats  did  not  vary  more  than  .03  of  a 
milligram  between  any  two  consecutive  experiments. 

The  Reduction  Apparatus. 

The  reduction  of  the  salt  to  metal  was  carried  out  in  an  ap- 
paratus which  is  most  easily  understood  by  means  of  the  ac- 
companying sketch.  Electrolytic  hydrogen  was  prepared  by 
passing  a  current  of  six  amperes  through  a  solution  of  sodium 
hydroxide.  The  platinum  electrodes  were  so  arranged  that 
no  oxygen  could  diffuse  into  the  hydrogen  compartment  and 
the  electrolyte  was  cooled  by  passing  water  through  a  coil  of 
glass  tubing  immersed  in  the  anode  compartment.  The  hydro- 
gen thus  generated  passed  through  a  wash  bottle  containing 
concentrated  sulphuric  acid,  then  over  red  hot  copper  to  re- 
move any  oxygen  or  hydrogen  peroxide  which  might  be  pres- 
ent. The  gas  next  passed  through  an  apparatus  made  en- 
tirely of  glass  composed  of  towers  containing  glass  balls.  In 
the  first  tower  the  balls  were  moistened  with  a  solution  of 
silver  nitrate.  The  second  and  third  contained  concentrated 
sulphuric  acid.  The  drying  was  then  completed  in  a  U  tube 
containing  phosphorus  pentoxide,  from  which  the  hydrogen 
passed  directly  to  the  combustion  tube. 

The  nitrogen  was  prepared  by  passing  ammonia  gas  and  air 
over  red  hot  copper.  The  excess  of  ammonia  was  absorbed 
by  passing  the  gas  through  three  wash  bottles  containing 
dilute  sulphuric  acid.  The  nitrogen  was  purified  by  passing 


Explanation  of  Cut. 

A.  Hpdrogen  generator. 

B.  Sulphoric  acid  wash  botttle 

C.  Combination  tubes  containing  copper  gauze. 

D.  Silver  nitric  tower. 

K.  and  F.  Sulphuric  acid  towers. 

G.  U.  tube  containing  phosphorus  pentoxide. 

H.  Electric  Heating  coil. 

I.  Combination  tube. 

J.  Sulphuric  Acid  trap. 

K.  Ammonia  flask. 

L,.  Air  supply. 

M.  N.     O.     Wash  bottles  containing  dilute  sulphuric  acid, 

P.  Silver  nitrate  tower. 

Q.  Tower  containing  concentrated  sulphuric  acid. 

R.  Fused  potassium  hydroxide  tower. 

S.  Tower  containing  phosphorus  pentoxide. 

T.  Trap  for  escape  of  nitrogen. 

W.  Water  for  cooling  coil. 

X.  100  volt  direct  current. 

Y.  Lamp  bank  resistance. 

Z.  Resistance  box. 

Am.     Amperemeter. 


Li-Li 


it  through  a  series  of  towers  constructed  entirely  of  glass  con- 
taining the  following  re-agents  in  the  order  mentioned :  a 
solution  of  silver  nitrate,  concentrated  sulphuric  acid,  fused 
potassium  hydroxide,  and  phosphorus  pentoxide.  From  the 
last  tower  the  gas  passed  to  the  two-way  stopcock  by  means 
of  which  it  was  admitted  to  the  combustion  tube  or  allowed  to 
escape  into  the  air  through  a  sulphuric  acid  trap.  This  trap 
was  necessary  in  order  to  entirely  free  the  apparatus  of  air 
while  the  hydrogen  was  passing  through  the  combustion  tube. 

The  Jena  tube  in  which  the  reduction  took  place  was  con- 
nected with  the  apparatus  by  a  carefully  ground  joint  which 
was  not  lubricated,  but  held  in  close  contact  by  a  rubber  band. 
The  farther  end  of  the  tube  was  supplied  with  a  sulphuric 
acid  trap  to  prevent  the  entrance  of  air. 

This  tube  was  heated  with  an  electric  heater  'made  by  cov- 
ering a  sheet  iron  tube  with  asbestos  then  winding  it  with 
6  feet  of  No.  32  platinum  wire.  The  whole  was  covered  with 
several  thicknesses  of  asbestos  to  prevent  loss  of  heat.  The 
current  supplied  the  heater  was  controlled  by  a  lamp  bank 
resistance  and  varied  from  0.25  to  1.6  of  an  ampere.  By  this 
means  the  temperature  could  be  raised  very  gradually  until 
the  salt  was  reduced  and  the  ammonium  compound  volatilized. 

Reduction  of  the  Pallado-Diammonium  Compounds. 

The  counter-balanced  boat  was  filled  with  salt  and  after 
standing  in  the  balance  room  for  two  hours  it  was  weighed. 
The  boat  and  its  contents  were  then  transferred  to  the  com- 
bustion tube  where  the  boat  rested  on  a  piece  of  platinum  foil. 
The  apparatus  was  then  closed,  the  hydrogen  generator 
started  and  0.25  ampere  allowed  to  flow  through  the  heating 
coil  for  two  hours.  The  current  was  then  raised  to  0.5  am- 
pere which  completed  the  reduction  in  another  hour.  The 
current  was  now  gradually  increased  to  1.6  amperes  which 
heated  the  boat  and  contents  to  955°.*  With  the  pallado- 
diammonium  chloride  the  metal  usually  came  to  constant 
weight  after  heating  for  five  hours.  The  hydrogen  and 
heating  currents  were  then  broken  and  nitrogen  passed 
through  the  tube  while  it  cooled,  which  required  about  one 
hour.  The  boat  was  then  placed  in  its  weighing  bottle  and 
weighed  after  it  attained  the  temperature  of  the  balance  room. 
This  weight  was  checked  by  heating  in  hydrogen  for  two 

*This  temperature  was  determined  by  melting  finely  divided  silver  in 
a  boat  placed  within  the  tube. 


10 

hours  and  cooling  in  nitrogen  as  before.     No  weight  was  ac- 
cepted which  did  not  check  to  0.05  milligram  or  less. 

The  reduction  of  the  double  cyanide  was  similar,  only  it 
required  8  to  10  hours  heating  in  hydrogen  to  bring  it  to  con- 
stant weight. 

By  this  method  the  reduction  and  volatilization  of  the 
ammonium  salts  took  place  very  slowly  so  that  there  was 
very  little  chance  of  any  palladium  being  carried  out  of  the 
boat  by  the  volatile  salts ;  but  in  order  to  be  sure  of  this,  the 
combustion  tube  was  washed  out  with  aqua  regia  and  water 
after  each  experiment,  and  the  solution  carefully  tested  for 
palladium,  but  always  with  negative  results. 

The  following  results  were  obtained  by  the  reduction  of 
salts  in  hydrogen,  using  the  following  values :  Chlorine  35.473, 
hydrogen  1.068,  nitrogen  14.01. 

Pallado-diammonium  chloride.     Pd(NH3Cl)2.     A. 

Weight  of  Salt.          Weight  of  Metal          Atomic  Weights. 

O  =  i6 

1.  0.89187          0.44885  106.40 

2.  0.77931  0.39218  106.38 

3.  0.66980  0.337II  106.41 

4.  1.08373  O.S454I  106.40 

5.  0.96048  0.48338  106.40 

Mean,  —     106.399 
Pallado-diammonium  chloride.     Pd(NH3Cl)2.     B. 

1.  0.95615  0.48129  106.43 

2.  0.94087  0.47356  106.42 

3.  0.90106  0.45353  106.42 

4.  1.16994  0.58908  106.50 


Mean,  —  106.442 

Pallado-diammonium  cyanide.     Pd(NH3CN)2.     C. 

1.  0.85860  0.47463  106.41 

2.  1.19378  0.66002  106.45 

3.  1.41818  0.78408  106.45 

4.  1.05254  0.58206  106.51 

5.  I-395IO  0.77153  106.51 

6.  1.66196  0.91881  106.42 

Mean,  —    106.458 
The  mean  of  the  15  determinations,     106.434 

The  Electro-chemical  Equivalent  of  Palladium. 
When  this  work  was  started  we  hoped  to  be  able  to  deter- 
mine the   electro-chemical   equivalent  of  palladium   as   com- 
pared to  silver.     Numerous  experiments  were  tried  using  the 


11 

Richards  type  coulometer,  but  the  results  were  never  concord- 
ant. About  this  time  L.  Wohler  and  F.  Martin1  stated 
that  palladium  used  as  the  anode  in  nitric  acid  solution  was 
oxidized  to  Pd2O3  and  PdO2.  This  explained  our  variable 
results  and  seems  to  prove  the  method  useless  for  atomic 
weight  work. 

Pallado-Diammonium  Chloride. 

When  this  salt  is  prepared  as  described  there  are  often  a 
few  dark  red  crystals  which  form  with  the  ordinary  light 
yellow  variety.  This  seems  to  be  only  a  dimorphous  form,  for 
the  red  crystals  are  not  soluble  in  water  and  they  disappear 
on  drying,  but  palladium  ammonium  chloride  (NH4)2PdCl4 
is  formed  by  the  addition  of  ammonium  chloride  to  a  solution 
of  palladous  chloride.  A  trace  of  it  might  be  formed  along 
with  the  pallado-diammonium  chloride,  since  the  excess  of  am- 
monia is  neutralized  with  hydrochloric  acid.  Such  does  not  Seem 
to  be  the  case  for  if  the  double  chloride  were  present  it  would 
tend  to  lower  the  per  cent,  of  palladium.  To  determine  this 
point  a  sample  of  the  diammonium  chloride  was  washed  with 
very  dilute  hydrochloric  acid  for  one  week  then  washed  with 
water  for  one  day.  After  drying  to  constant  weight  the  per 
cent,  of  palladium  was  slightly  higher  than  from  the  unwashed 
samples. 

i     Wohler  and  Martin,  Chem.  Zeit.  March  25,  1908. 

Conclusions. 

The  results  of  these  fifteen  determinations  give  a  mean  of 
106.434,  which  is  slightly  lower  than  most  previous  determina- 
tions. The  metal  was  very  carefully  purified  so  that  it  seems 
improbable  that  the  difference  could  be  accounted  for  here. 
In  the  reductions  the  metal  was  heated  to  a  much  higher 
temperature,  955°C,  than  by  previous  investigators.  This  may 
have  expelled  the  ammonium  chloride  more  completely,  but 
it  does  not  seem  sufficient  to  account  for  the  difference.  Var- 
iations in  th  pallado-diammonium  chloride  seem  to  be  a  pos- 
sible source  of  difference. 

It  is  hoped  that  the  work  may  be  continued,  determining  the 
constancy  of  pallado-diammonium  chloride,  using  metal  from 
different  sources  and  determining  new  ratios,  if  possible. 


UNIVERSITY 

OF 


